Error: Command ‘check-host-alive’ has already been defined
Error: Could not register command (config file ‘/etc/nagios/minimal.cfg’, starting on line 76)

***> One or more problems was encountered while processing the config files…

Check your configuration file(s) to ensure that they contain valid
directives and data defintions. If you are upgrading from a previous
version of Nagios, you should be aware that some variables/definitions
may have been removed or modified in this version. Make sure to read
the HTML documentation regarding the config files, as well as the
'Whats New' section to find out what has changed.

I had this problem too. Several commands are defined in both the checkcommands.cfg and minimal.cfg. At first I commented out the duplicates in minimal.cfg, and then ended up removing it altogether so I could use separate cfg files for hosts, services, etc.

empty the etc directory (keep backup)
create a minimal.cfg file with only one host, one service, ona contactgroup and one contact.
when it works divide into more files to keep things clean.
whent this one works add more services and hosts… one at a time